Preparation and dielectric properties of polyimide foams containing crosslinked structures

In order to obtain cellular materials with low dielectric properties, crosslinked polyimide foams were prepared using 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride (BTDA), 4,4′‐oxydianiline (ODA) and 2,4,6‐triaminopyrimidine (TAP) as monomer via a poly(ester‐amine salt) precursor process. The structures of the precursors and the polyimide foams were characterized by thermogravimetric analysis (TGA) and FT‐IR, while the morphologies of the polyimide foams were viewed from scanning electron microscopy (SEM) measurements. The results revealed that the poly(ester‐amine salt) precursor containing TAP could successfully be converted to a crosslinked polyimide foam with relatively uniform cell structure. Also, the crosslinking of TAP improved the mechanical properties of foams in comparison with the non‐crosslinking systems. With increasing content of TAP, the dielectric constants of the polyimide foams decreased gradually. For the foam with TAP molar ratio at 15%, the dielectric constant was as low as 1.77 at the frequency of 10 kHz. Though the thermal resistance decreased slightly for crosslinked foams, the decomposition temperatures were still maintained above 520°C. Copyright © 2006 John Wiley & Sons, Ltd.     

Electrical and dielectric properties in carbon fiber‐filled LMWPE/UHMWPE composites with different blend ratios

Carbon fiber (CF) filled low‐molecular‐weight polyethylene (LMWPE) and ultra‐high molecular weight polyethylene (UHMWPE) composites were prepared by the gelation from solution and the kneading in the melting state. The content of carbon fibers was fixed to be 23.5 vol %. The resistivity, positive temperature coefficient (PTC), and dielectric behaviors of the composites became more pronounced with increasing content of LMWPE with much higher thermal expansion than that of UHMWPE. The PTC effect became most significant, when the blend ratio of LMWPE to UHMWPE was 9/1. Beyond 9/1, the PTC effect was less pronounced. Scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) revealed that the UHMWPE and LMWPE chains within the composite crystallized independently by gelation from solution and were virtually unaffected by the presence of carbon fibers. Consequently, it was confirmed that carbon fibers selectively were localized in the mixed region of LMWPE and UHMWPE for the composite (3/1 and 6/1) and mainly in the region of LMWPE for the 9/1, 12/1, and 15/1 composites. This indicated that the content of carbon fibers within LMWPE region was the highest for the 9/1 composite and the 9/1 composite provides the most significant PTC effect. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 359–369, 2008     

Synthesis and properties of ultralow dielectric porous polyimide films containing adamantane

A series of novel ultralow dielectric porous polyimide (PI) films containing adamantane groups was prepared via the thermolysis of polyethylene glycol (PEG) oligomers mixed into PI matrix. Scanning electron microscopy results indicated that the porous PI films showed closed pores with an average diameter of 120 ± 10 nm. Good thermal properties with 5% weight loss temperature of 499 °C in air atmosphere and glass transition temperature in excess of 310 °C were shown for porous PI films. Notably, the ultralow dielectric constant of porous PI films with 1.85 at 1 MHz was obtained and revealed via broadband dielectric spectroscopy. The effects of the chemical structure of the PI matrix and PEG content on the decomposition behavior of PEG and the performance of porous films were investigated. Wide‐angle X‐ray diffraction results indicated that the PI matrix with large d‐spacing generated weaker interactions between the PEG and PI backbone than those of PI matrix with small d‐spacing. As a result, the PEG for the PI matrix with large d‐spacing was completely decomposed. As indicated by the broadband dielectric spectroscopy results, lower dielectric porous PI films were prepared when the PEG contents in the PI matrix increased from 0 to 20 wt %. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 549–559     

Reverse mode operation polymer dispersed liquid crystal with a positive dielectric anisotropy liquid crystal

The development of electrically activated chromogenic materials is important for their potential applications in smart windows. Several previous works have reported on reverse mode operation polymer dispersed liquid crystals (PDLCs) based on negative dielectric anisotropy liquid crystals. They have a transparent OFF state, which turns opaque after the application of a suitable external electric field. Nevertheless, these devices have some limitations such as the use of large amount of expensive liquid crystals with peculiar physical‐chemical properties. In addition, a good matching between the refractive index of liquid crystal and the polymer matrix one is required. The main result of this work is the achievement of reverse mode operation devices prepared with a positive dielectric anisotropy liquid crystal and characterized by a high OFF state transmittance obtained by the onset of high intensity built‐in DC electric fields in a direct mode operation PDLC, which allows the OFF state homeotropic alignment of liquid crystal directors. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Mechanical and dielectric properties of segmented polyurethane elastomers containing chemical crosslinks in the hard segment

Mechanical and dielectric properties of two series of segmented polyurethanes having soft segment concentration of 50 and 70% and a varying degree of crosslinking through the hard segment were studied. The degree of crosslinking in each series was varied by varying the butane diol/trimethylol propane ratio in the chain extender mixture. Tensile strength, elongation at break decrease, but elastic recovery increases monotonically with increasing crosslinking. The plateau modulus in the dynamic mechanical test decreases and then increases with increasing TMP content. Crosslinking causes broadening of the soft segment glass transition as seen by permittivity and loss factor measurements. It also affects high temperature behavior (above the glass transition of the hard segment); it lowers permittivity, loss factor, and ionic conductivity. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 237–251, 1998     

Effect of fulleroid materials on the mechanical and tribological properties and dielectric relaxation of polyamide 6 nanocomposites

The effect of fulleroid materials (fullerene С₆₀ and fullerene soot, which is used for fullerenes production) on the mechanical and tribological properties of polymer nanocomposites based on polyamide 6 (PA6) was investigated. Composites were synthesized by direct mixing in an extruder. The use of the nanoparticles was an effective way to decrease the friction coefficient of the polymer composites because the fillers had the same size as the segments of the surrounding polymer chains. The steady state coefficients of friction with addition of fulleroid fillers were lower than that of unfilled PA6. The lowest coefficient of friction was observed for PA6 filled with 1 wt% fullerene soot. Dielectric spectroscopy was used to investigate the influence of nanoparticles on the relaxation processes in the polymer matrix. It is found that the segmental relaxation processes become faster with the addition of fullerene С₆₀. In contrast, the secondary processes of PA6/fullerene C₆₀ nanocomposites were observed to slow down with the addition of fullerene C₆₀. This means that the local “molecular stiffness” is increased, and a phenomenological link between the secondary relaxation times and the mechanical properties explains the increase in the Young's modules of the nanocomposites upon the addition of С₆₀. These observations suggest that nanoparticles can have a qualitatively different effect on the matrix polymer dynamics at different length scales, and caution must be taken in comparing the changes in the dynamics associated with different relaxation processes. Copyright © 2016 John Wiley & Sons, Ltd.     

Morphology and dielectric properties of an epoxy network modified by end‐functionalized liquid polybutadiene

Epoxy resin networks modified with different functionalized liquid polybutadiene were characterized by scanning electron microscopy, atomic force microscopy (AFM), and dielectric thermal analysis techniques. Different morphologies were observed for these different systems, which were attributed to different interaction degrees between the components. Hydroxyl‐terminated polybutadiene (HTPB) and carboxyl‐ terminated polybutadiene (CTPB) resulted in epoxy networks with two‐phase morphology that differed in rubber particle size. The use of isocyanate‐terminated polybutadiene (NCOTPB) resulted in transparent thermoset material, whose rubber domains were in the nanoscale dimension, only detected by the AFM technique. The different morphological aspects in these epoxy systems also affected the dielectric properties. The epoxy–HTPB network exhibited two low temperature relaxation peaks corresponding to two different phases present in the system, whereas the epoxy–CTPB or epoxy–NCOTPB systems, whose rubber particles are well adhered to the epoxy matrix by chemical bonds, displayed only one single low temperature relaxation peak. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4053–4062, 2004     

Preparation and dielectric properties of composites based on PVDF and PVDF-grafted graphene obtained from electrospinning-hot pressing method

In this work, graphene was modified by the grafting of poly(vinylidene fluoride) (PVDF) using Friedel-Crafts reaction to form PVDF-grafted graphene (PGG) filler, which was confirmed by transmission electron microscope, Fourier transform infrared, Raman spectroscopy, wide angle X-ray diffraction, and thermogravimetric analysis. The solution containing as-prepared PVDF-grafted graphene and PVDF was electrospun to form fibrous membranes, which was subjected to hot pressing in the laminating mode to prepare solid PGG/PVDF composite films. The structures of electrospun fibrous membrane and solid PGG/PVDF composite film were investigated by scanning electron microscope. Furthermore, it could be found that the dielectric constants of PGG/PVDF composites exhibiting relatively low dielectric loss factors were significantly higher than that of pure PVDF.     

Influence of particle size and particle loading on mechanical and dielectric properties of biochar particulate-reinforced polymer nanocomposites

This research work emphasizes using pulverized biochar obtained by the pyrolysis of rice husk as a particulate reinforcement in unsaturated polyester matrix. The influence of particle size and particle loading on the mechanical and dielectric properties of particulate composites were investigated. The mean size of particles obtained through pulverizing using ball mill varied from 510 to 45 nm when milled for a duration ranging from 6 to 30 h. The particle loading in the composite varied from 0.5 to 2.5 wt%. The impact strength of the specimen having particle loading of 2.5 wt% with 45 nm particle size increased by 77.50%, and its dielectric constant increased by 7% when compared to that of cured pure resin; however, the tensile strength decreased. The biochar particles were subjected to X-ray diffraction, Fourier transform infrared spectroscopy (FT-IR), and atomic force microscopy (AFM) analysis for characterization. Morphological studies were performed on tested samples by scanning electron microscope.     

Synthesis,characterization, and thermal and dielectric properties of three different methacrylate polymers with zwitterionic pendant groups

The synthesis, characterization, thermal, and dielectric properties of three different zwitterionic methacrylates of the sulfobetaine type are presented. Diethylamine-ethyl-, 2-(diethylaminoethoxy)-ethyl-, and 2-(2-diethylaminoethoxy) ethoxy-ethyl-methacrylates were made to react with butanosultone to prepare monomers with variable flexibility. The flexibility of the lateral chain of the polymethacrylates decreased the glass transition temperature (T_g down to 300 K) of the polymers. A linear relationship between T_g and the number of carbon atoms was shown for these materials. X-ray diffraction and DSC experiments showed the formation of new ordered phases in these polymers, which inhibited their dipole conductivity. On heating, these phases were destroyed and values of conductivity of 10⁻⁷–10⁻³ S cm⁻¹ were obtained in the studied range of temperature. Variation of conductivity with temperature was established according to the Arrhenius equation. Dielectric properties exhibited a small deviation of the Debye type behavior, and β parameters of the Cole–Cole equations were calculated for the synthesized polymers. © 1997 John Wiley & Sons, Inc.     

The dielectric properties of dry and water-saturated nylon-12

The dielectric relaxation of 30% crystalline Nylon-12 of molecular weight 20,000 has been studied at frequencies from 12 Hz to 0.1 MHz and temperatures from 77 to 375 K, and the effect of water on the relaxation spectra has been investigated. Absorbed water increases both the rate and the intensity of both its α-and β-relaxation processes and increases the rate but decreases the intensity of its γ-relaxation process. These results are interpreted in terms of the hydrogen bonding effects of water on localized motions of dipoles in Nylon-12. The relatively large half-width of the α-relaxation, which becomes better resolved at high temperatures, is attributed mainly to the random distribution of crystalline regions in the polymer. It is suggested that water lowers the steric hindrance for the localized mode of dipolar reorientation and causes a redistribution of local sites such that the β-relaxation process grows at the expense of the γ-process.     

The formation of percolative composites with a high dielectric constant and high conductivity

The dielectric constant and electrical conductivity of a composite of two insulators, poly(1,1-difluoroethylene) (yellow) and K(2) CO(3) (white), increased dramatically near the percolation threshold?f(c) (f=concentration of K(2) CO(3) ). This intriguing phenomenon can be interpreted in terms of interface percolation caused by the formation of chemically activated interfaces.     

Synthesis and characterization of unsymmetrical benzonitrile‐containing polyimides: Viscosity‐lowering effect and dielectric properties

A new unsymmetrical diamine, 2‐(3‐aminophenoxy)‐6‐(4‐aminophenoxy)benzonitrile (3,4‐APBN), is synthesized via two consecutive S_NAr reactions and the temperature‐dependent reactivity of the fluorides in 2,6‐difluorobenzonitrile, whose first S_NAr reaction occurs at 70 °C and second, at 100 °C, allowing timing control of reaction sequence and circumventing the transetherification side reaction. Thus, a series of polyimides (PIs) is prepared from the polymerization of 3,4‐APBN with five common dianhydrides (6FDA, DSDA, OPDA, BTDA, and PMDA). For comparison, a second series is also prepared from two symmetrical diamines ([2,6‐bis(3‐aminophenoxy)benzonitrile (3,3‐APBN) and 2,6‐bis(4‐aminophenoxy)benzonitrile (4,4‐APBN)] and 6FDA or PMDA. The processability of the poly(amic acids) (PAAs), for the first series is greatly improved since their solution viscosities are much lower than PAAs based on symmetrical diamines. Besides having high glass‐transition temperatures (249–332 °C), and thermal stability [5% weight loss in the range of 505–542 °C (air) and 512–546 °C (nitrogen)], these PIs form tough, transparent and flexible films that have a tensile‐strength range of 82.1–121.3 MPa, elongations‐at‐break of 5.33–9.81%, and tensile moduli of 2.11–2.97 GPa. Their film dielectric constants are 3.08–3.62 at 10 kHz, moderately higher than that (2.92) of analogous PI (CP2) without nitrile groups. Overall, we found that the reduction of structural symmetry in repeat units can improve the polymer processibility as well as increasing their dielectric constants. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4998–5011     

Enhanced dielectric performance of a block copolymer‐polythiophene nanocomposite

Dielectric elastomer actuators (DEAs) transform electrical energy into mechanical work. However, despite displaying exceptional features, the low permittivity of elastomers restricts their application. Hence, to overcome this limitation, DEAs are fabricated by dispersing poly(3‐methylthiophene acetate) (P3TMA), a polarizable conducting polymer, into poly[styrene‐b‐(ethylene‐co‐butylene)‐b‐styrene] (SEBS), a thermoplastic elastomer with excellent mechanical properties. Although high‐quality SEBS:P3TMA films are obtained for all compositions (between 0.5 and 20 wt % P3TMA), their thickness and surface roughness increase with the nano‐sized filler content. Moreover, the conducting particles are well integrated into the SEBS network with no evidence of aggregation or significant change in the mechanical properties of the composites. P3TMA, which forms encapsulated conductive domains within the polymeric matrix, improves the dielectric behavior of SEBS:P3TMA by increasing their dielectric constant with low dielectric losses and no current leakage. Thus, indicating the potential future application of these nanocomposites as elastomer actuators or high energy density capacitors. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1896–1905     

Thermomechanical analysis of a polymer dispersed liquid crystal containing a thermoplastic elastomer

Bulk samples of polymer dispersed liquid crystals (PDLCs) containing polystyrene (PS) and a thermoelastic elastomer as polymer matrices, have been prepared by a thermally-induced phase separation method. Thermomechanical analysis measurements revealed that the PDLC containing the thermoplastic elastomer possessed rubber-like elasticity even in the mesomorphic temperature range of the LC while the PDLC based on PS showed plastic deformation during the measurement.     

Thermomechanical analysis of a polymer dispersed liquid crystal containing a thermoplastic elastomer

Bulk samples of polymer dispersed liquid crystals (PDLCs) containing polystyrene (PS) and a thermoelastic elastomer as polymer matrices, have been prepared by a thermally-induced phase separation method. Thermomechanical analysis measurements revealed that the PDLC containing the thermoplastic elastomer possessed rubber-like elasticity even in the mesomorphic temperature range of the LC while the PDLC based on PS showed plastic deformation during the measurement.     

Theoretical investigation of polymer molecular structure influence on dielectric properties and mechanical properties

Triboelectric nanogenerator (TENG) technologies have explosive development in the field of energy harvesting and self-powered sensing. As the key element of triboelectric devices, dielectric polymers have obtained much attention in recent years. The dielectric properties of polymer determine the output performance of TENG. In this paper, we take silicone rubber as an example of dielectric polymers, to study the properties of molecular structure influence on the dielectric properties and mechanical properties by the molecular dynamics simulation method. The free volume fraction, dielectric constant, and mechanical properties of silicone rubbers with different branch chains were calculated. The dielectric constant is highly related to the free volume distribution and the dipole moments of silicone rubbers with different amounts of branch chains. For fewer branch chains silicone rubber, the free volume distribution contributes most to the dielectric constant; for more branch chains silicone rubber, the dipole moment dominates the dielectric constant. Therefore, the silicone rubber ratio has a great influence on the dielectric constant of silicone rubber. With the increase of temperature, the dielectric constant of 2-chain silicone rubber increases at first and then decreases, and the maximum value is obtained near 300 K. Therefore, it is necessary to control the temperature when silicone rubber is used as a dielectric material. This work can be a guide for improving the dielectric properties of silicone rubber, and it provides a new approach to the optimal design of high-performance triboelectric nanogenerators.     

Synthesis and properties of a cyanate ester containing dicyclopentadiene(II)

A 2,6‐dimethyl phenol‐dicyclopentadiene novolac (DCPDNO) was synthesized from dicyclopentadiene and 2,6‐dimethyl phenol, and the resultant DCPDNO was reacted with cyanogen bromide into 2,6‐dimethyl phenol‐dicyclopentadiene cyanate ester (DCPDCY). The structures of the novolac and cyanate ester were confirmed with Fourier transform infrared spectroscopy, elemental analysis, mass spectrometry (MS), and nuclear magnetic resonance. For the purpose of increasing the mobility of residual DCPDCY during the final stage of curing and achieving a complete reaction of cyanate groups, a small quantity of a monofunctional cyanate ester, 4‐tert‐butylphenol cyanate ester (4TPCY), was added to DCPDCY to form the cyanate ester copolymer. The synthesized DCPDCY was then cured with 4TPCY at various molar ratios. The thermal properties of the cured cyanate ester resins were studied with dynamic mechanical analysis, dielectric analysis, and thermogravimetric analysis. These data were compared with those of the commercial bisphenol A cyanate ester system. Compared with the bisphenol A cyanate ester system, the cured DCPDCY resins exhibited lower dielectric constants (2.52–2.67 at 1 GHz), dissipation factors (0.0054–0.0087 at 1 GHz), glass‐transition temperatures (261–273 °C), thermal stability (5% degradation temperature at 406–450 °C), thermal expansion coefficients (4.8–5.78 × 10^?5/°C before the glass‐transition temperature), and moisture absorption (0.8–1.1%). © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 671–681, 2005     

Optical and dielectric properties of new azobenzene copolyethers embedded in polymer matrices

A new group of photochromic azobenzene copolyethers was synthesized. Their photochomic and dielectric properties were studied in poly(vinyl alcohol) and poly(methyl methacrylate) matrices.